The invention relates to bilateral static controlled power switches in general, and more particularly to those of the forced commutation type, used where flow of power, AC or DC, may be occurring in opposite directions.
In a bilateral thyristor switch, two thyristors are provided, each associated with one direction of power flow. One of them is to be "open" whenever interruption is required for the particular direction. This function is particularly important in an uninterruptible power system (UPS) installation. In a UPS installation, energy is supplied to a critical load by a group of redundant inverter power supplies.
Static isolators are used in conjunction with the static inverter AC power supplies, mainly when a number of such power supplies operate in parallel feeding a common output bus in a UPS (Uninterruptible Power Supply) system.
Such systems are intended to provide AC power to a "critical bus", feeding loads that cannot stand power outages such as can occur with a regular commercial power source. In case of power outage, the critical bus remains energized through the UPS system, which then draws its input power from storage batteries. Reliability of operation of the UPS system is an essential requirement. For this reason, the UPS system is sometimes composed of a number of separate inverter units, operating in parallel. When more units are provided than required to meet the system total rating requirements, the system is said to have "redundancy". Each inverter is redundant, meaning that in case of failure of one inverter unit, it can be disconnected from the critical bus without impairing the system's rating.
In order to separate a failing inverter from the critical bus (thereby preventing it to draw damaging fault current from the other units and jeopardizing the supply of power to the critical load) efficient disconnect facilities must be provided by switches. Electromechanical disconnect switches are not adequate for this function, because their inherently slow response makes it improbable to limit below damaging levels the fault current drawn from the still healthy inverter units. Similarly, fused disconnect arrangements are too slow, unless means for forced fuse clearing are used; however, even in this case, the fact that a disconnection by fuse clearing is not resettable is an objection.
The isolator switch on each power supply must be capable of providing a rapid disconnection of a faulty inverter in order to protect the bus line. Forced commutation is in order for such prompt interruption. At the same time, low cost requires to avoid passive elements in the power path. In all events reliability is essential, which calls for a simple activation procedure. These goals have not been totally achieved in the past.
In AC systems, one of the ways of implementing the switching function via static semiconductor devices consists of using a pair of back-to-back thyristors which, when effecting the function of a switch in the ON (connecting) state, have their gates energized by a steady forward gate drive signal. When it is desired to turn the switch into the OFF state, the gate drive is suppressed to both thyristors. The device that was conducting current at the time of gate signal removal will keep conducting until the current through it reverses polarity due to the AC nature of the power source. Following this moment, the antiparallel thryistor device which would normally assume conduction of the load current when the isolator switch is in the ON state, will fail to do so due to the absence of gate drive, causing the circuit to become open. Such system is said to be a "naturally commutated" static switch. The great simplicity inherent to this arrangement is counterbalanced by the existence of a response delay, due to the need of waiting until the current undergoes a natural zero crossover. This delay, which could at worst last as long as the half-period of the AC current wave, is in some cases excessive to warrant safe disconnection of a faulty inverter. Although significantly improving over the electromechanical switch situation, the naturally commutated static switch of FIG. 2 may still not be fast enough in a number of fault situations.
The two back-to-back thyristors could be replaced with semiconductor devices that can be turned off through a control electrode, such as transistors or GTO (gate turn off) thyristors. At any instant, the switch could then be quickly brought to the OFF state by properly driving the control electrode. However, this solution is presently not applicable to the case of high power static isolators due to the lack of semiconductor devices of adequate voltage and current rating.
Alternatively, it is known to associate the thyristors with additional circuitry allowing to artificially and rapidly reduce to zero the current through a conducting device at any desired moment, thereby extinguishing the device. This is the process of forced commutation of a thyristor. In force commutated static isolators, the time delay inconvenience inherent of naturally commutated static isolators is eliminated at the expense of added circuitry to obtain the forced commutation effect and at the expense of providing thyristors apt to be quickly extinguished. The novel circuit configuration according to the present invention implements a force commutated static isolator.
In practice, from the standpoint of the speed of power flow interruption, the function of the switch is best implemented through static means, i.e., by using semiconductor devices that are brought to their conducting state when the switch is supposed to be closed and caused to block the flow of load current when the switch is supposed to be open. The semiconductor devices' arrangement implementing the switching function is called here a "static isolator".
Forced commutation is known in two forms, "hard"and "soft". While "hard" commutation implies that subsequent to a rapid forcing of the current to zero, a substantial reverse voltage is applied across the thyristor, "soft" commutation supposes that a conducting thyristor is turned off by gradually reducing its forward current to zero at a rate well below the thyristor di/dt rating, and then applying a very modest reverse voltage to the extinguishing thyristor. Such low reverse voltage is generally the forward drop across a device in antiparallel connection with the thyristor. Hard commutation calls for connecting a precharged commutating capacitor across the device by means of gating an auxiliary switching device. Current then is commutated from the thyristor into the capacitor branch at a rate controlled by the inductor L in the circuit. When the thyristor ceases to conduct, the "hard" capacitor voltage appears in reverse across the thyristor and in series with the system voltage, resulting in the first drawback of hard commutation: since the system voltage is boosted by the capacitor voltage, the fault current rises even faster (and therefore higher) until the capacitor voltage reverses. A second drawback of hard commmutation in this basic form is that the load impedance starts depleting capacitor charge from the moment the commutating branch is activated. Consequently, a larger capacitor is required to provide the necessary reverse bias time. Another drawback of hard commutating circuits is that inductance in circuit has the function of limiting rate-of-rise of current (di/dt) through the discharge circuit and not to "tune" the discharge circuit. The presence of an inductance actually "degrades" the circuit since reverse biasing of the thyristor does not start until after the entire line current is commutated by the capacitor; and by this time a non-negligible charge has been depleted.
Typical of an isolator switch interrupted by "hard" forced commutation in the prior art are the circuits shown in U.S. Pat. Nos. 3,737,759 and 3,921,038.
An object of the present invention is to overcome the above-stated drawbacks due to "hard" commutation.
Another object of the invention is to also provide a bilateral static isolator which is commanded by "soft" forced commutation.
A further object of the invention is to provide a force commutated static isolator that can be used in an uninterruptible power system to isolate any particular inverter of such system.
A further object is to provide an isolator without requirement of elaborate signal sensing prior to activation.
The technical and commercial constraints specific to a UPS make it important that the circuit arrangement to be used have as many as possible of the following characteristics: The number of power components must be small in order to ensure reliability. The number of semiconductor junctions bearing current in continuous duty should be kept to a minimum to reduce the power losses and the cooling requirements. Passive components in the path of the steady-state load current should be avoided, to eliminate the cost penalty, the losses and the voltage drop associated with such practice. The mechanism of activation of the circuit should have a rugged simplicity for the sake of the system's reliability. This requirement of reliability actually dominates all other considerations since the static isolator, as part of a UPS system, must in the final analysis ensure an overall reliable source of power. In this respect, the prerequisite of reliability in a static isolator has two aspects: not only must the system be capable of performing its isolating function when required to do so, but it must also be immune to untimely, erratic triggering when no disconnect action is desired, i.e., under normal operation and in absence of fault conditions. As a matter of fact, untimely activation of a static isolator due to malfunction in the control circuitry may well occur with a far higher probability than the failure of the isolator itself in performing its protective function. Since the unintentional activation of static isolators can result in the collapse of the critical bus voltage, it is an unacceptable occurrence for a UPS system. For this reason, it is desired to decrease the isolator's vulnerability to control failure and adopt a system that does not require elaborate current sensing the decision making logic to perform its intended function.